text:
Why should jet engine throttles be moved smoothly?
On the only jet engined aircraft I'm trained on, the Bell 206B (Allison 250 engine), I was taught to move the throttle slowly and smoothly, especially when opening.  I've also see this done with Rolls Royce testing RB211s and early Trents.
I've realised I don't actually know why this is.  Is it to prevent possible flame out with a sudden increase in fuel with no corresponding increase in air mass since the engine will spool up much more slowly?
Do modern engines need this handling?  In this video at about 0:17, the pilot commences the roll by more or less slamming the throttle open. My guess is that like all other computer controlled controls, the FADEC interprets this as "as much fuel as possible without flaming out".

It's worth noting that moving the throttle smoothly is not exclusive to jet engines: there are reasons you want to move the throttle smoothly in piston aircraft too  (also here for some more general "be kind to your engine" reasons). The results on jets just tend to be more immediate and dramatic as casey noted.

Yeah, it's pointless to slam the throttle to the stops in a tenth of a second when it could take 5 or more seconds to go from flight idle to TO power. As I was taught at racing school, the car can't accelerate as fast as you can move your gas pedal foot so just chill out.

If you're Boeing (working on the first 747), sometimes you do. If Pratt and Whitney aren't taking your flame-outs seriously, you take their president up, blow two engines and then ask if they want to go for broke?

@Mazura The video in your link appears to have been removed due to a copyright claim.

The video was: Smithsonian Channel 747 The Jumbo Revolution

The issue is the possibility of a compressor stall.  The combustion will directly respond to changes in the thrust lever but it takes time for this to spool up the turbine that is connected to the compressor.  If the incoming air from the compressor is not of adequate pressure because it hasn't spooled up enough you risk a compressor surge.  In these engines you need to slowly increase thrust so that the turbine can spool up (and hence the compressor) so the airflow through the engine remains adequate for the engine power.
Modern engines with FADEC systems should not have these issues as the FADEC can enforce rate limits for fuel flow increases or directly calculate the maximum safe fuel flow from observed engine parameters.  Modern engines might also have more sophisticated compressors with variable stators that can help mitigate compressor stalls/surges.

I'm torn.  I accepted your answer but I must say that Peter's is I think more complete.

@Simon: A) Accepting the first answer, especially around here, isn't always the best policy, and B) you can uncheck Casey's answer and select Peter's if you feel it helps you more - SE is about the answer that best helps the questioner, mythical, magical unicorn-based Internet points shouldn't factor into the check-mark decision. (Nothing personal, of course, Casey - great answer!)

@Simon accept whichever answer you feel is most useful to you and solves your problem best.  Neither of us is going to fret over 15 rep, so don't worry about it.

Variable stators aren't a new thing; GE was using them all the way back in the 1950s.

Compressor stall is only one reason. Casey is right: Moving the throttle too quickly leads to an oversupply of fuel which in turn will add too much energy to the air in the combustor, heat it and let it expand more quickly than what the turbine will accept. This will increase the pressure in the combustor over that in the last stage of the compressor, and the heated air will escape the combustor in both directions. That is a fancy way of saying that the compressor stalls.
However, the oversupply of fuel in the combustor will also overheat the engine components. Given that jet combustors run rather lean, there would be plenty of oxygen left to burn the extra fuel. Temperature would rise quickly before the air mass flow can help to cool the combustor. Both the combustor and the turbine would overheat, and in the worst case the engine would be destroyed. Modern engines use computer control to increase fuel supply only slowly. Before that, the pilot had to be careful not to move the thrust levers forward from idle too quickly.
An added risk on multi-spool engines is compressor surge from a dynamic imbalance. The lower inertia of the high-pressure spool allows it to speed ahead of the low-pressure spool, and now the later stages of the compressor run too quickly and are starved of air, because the low-pressure part cannot pump enough air to keep the high-pressure compressor from stalling. Only careful application of thrust commands or computer control can limit the extra heat in the turbine such that the high-pressure compressor will stay within its surge limits.

This makes complete sense, and also explains why during the start procedure on the Allinson, you watch the TOT like a hawk.  If the limits are busted, you chop the fuel and continue cranking.

Shouldn't the FADEC prevent this?

@rbp - Where one exists, yes; the FADEC essentially serves as a "fly-by-wire" control for the jet engine. However, not all jets have them, though practically all commercial airliners currently flying do have one as it eliminates the flight engineer requirement.

Then the answer for practical purposes in commercial air travel is that the pilot  doesn't have to ease the thrust lever forward to prevent a compressor stall.

@rbp I don't know if any Jet Rangers have FADECS, but the ones I flew did not.  The FADEC was the slab of wet meat holding the throttle.

Also, with rotorcraft, you have the additional problem of the huge moment of inertia hanging off the power turbine, delaying acceleration.  Even though the gas generator is not on the same shaft, they are aerodyamically coupled:(

Yes for engines without FADECs, the FADEC am won't do anything. Neither will the mixture, alternate air, or carb heat work if those are not installed.


text:
What does it mean when an airplane has the "super" call sign?
What does it mean if an airplane uses the "super" designation after their call sign on the radio?

It's interesting that this was answered in a lot more detail months ago and yet this question and its answers got a lot more votes. - haha

@reirab Hot Network Questions at work.

@reirab It's also interesting that I searched for the question and didn't find the one that is referenced as its duplicate before asking. The word "duplicate" also gets applied pretty liberally on SE, but if someone lands on this question I agree that they'll get the most comprehensive answer by also reading that question and its answers.

@RyanBurnette Right, that's the point of 'duplicates' on SE. Unlike most other types of closed questions, questions that are closed as duplicates are useful in that they help people who are searching find the answer they're looking for, even when their search might not have found the original question, but it still leaves all of the information in one place, rather than fragmenting it. Having a question closed as a dupe isn't a bad thing at all, especially in cases like this where you actually did search and didn't find the answer.

@RyanBurnette I found the other question because I just happened to remember that this had been discussed (probably more than once) before, already knew the answer, and specifically knew to look for "wake turbulence."

@reirab By the way, you helped me understand the value of marking questions as duplicate. Up until this point I view duplicate as a bad thing. Now I think it usually enriches SE overall.

According to ICAO wake turbulence categories, it is a super-heavy aircraft capable of generating correspondingly super-strong wake vortices. Currently the only "super" aircraft are the Airbus A380 and Antonov An-225 (FAA).

Is there a reason the An-225 wouldn't be in the Super category for wake turbulence separation?

Looks like according to the FAA it is; it wasn't originally, as Super was created after the A380 first flew.

It's an indication not only of the type, but also the load.  An empty aircraft with minimum fuel will generate less wake turbulence than the same type at max landing weight.

@DouglasHeld it's true that a lighter aircraft will generate less wake turbulence, but only the maximum takeoff weight of the type is considered for the wake turbulence categories and callsigns.

The aircraft is either an A380, or AN225. Reference

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
How are cloud bases and tops reported on PIREPS?
Let's say I read a PIREP as follows...
KATL UA/OV KATL/TM 1522/FL080/TP C210/SK SCT013-TOP079/WX FV05SM
 HZ/TA M04/WV 24540KT/TB LGT/RM IN CLR.

I'm in KATL where the field elevation is 1,026, so if I were reading a METAR that reported bases at 013 I'd expect to enter the clouds at 2,300' MSL. I'm reading a PIREP here though, so based on this am I expecting to enter the bases as 1,300 MSL or 1,300 AGL? I have the same question with the tops.

KATL in a small plane? I feel for ya' man...

It depends. For a PIREP cloud Heights are reported as they are read by the pilot.
FAR AIM 7-1-16(b)

... In reports disseminated as PIREPs, height references are given the same as received from pilots, that is, above MSL.

This means some conditions may be reported MSL as read of the altimeter at local barometer settings while others may be set at standard pressure and read from there. I would assume that altitude reports less than 18,000ft are MSL.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How strong must wind gusts be to prevent landings and takeoffs?
I hope this is not as trivial as it sounds. Is there a wind gusts threshold based on which, at a given airport, incoming flights are diverted and outgoing ones are delayed?
I assume that aircraft are designed to handle a specific maximum wind gust, so if there is any such threshold, I expect it to be less than the design threshold. Is this correct?

You have two questions here,

I hope this is not as trivial as it sounds. Is there a wind gusts
  threshold based on which, at a given airport, incoming flights are
  diverted and outgoing ones are delayed?

As far as I know there is no specific threshold for which they will divert. You may want to read up on wind shear here as an airport may advise sever or unsafe wind shear and urge pilots to divert. There will be a point (often at single runway airports) where a landing in a particular type of aircraft becomes unsafe or even impossible as a result of wind but this will vary by plane.

Also, I assume that aircraft are designed according to a specific max
  wind gust, so if there is any wind gusts threshold, I expect it to be
  less than the design threshold. Is this correct?

Yes, aircraft are designed with whats called a maximum demonstrated crosswind component. The key here is that this is a demonstrated limit not necessarily an airframe limitation. The number is published in the POH for the aircraft made after 1975 and provides a general guideline of the maximum cross wind the plane can be landed safely in however its more than possible to land it in higher wind situations.
Also it should be noted that some of this is up to the pilot in command. Just because a towered airport clears you to land does not mean it is safe for you to do so likewise just because a pilot diverts does not mean they were instructed to do so by the tower they may self elect based on their personal abilities.

Probably worth adding that the aircraft operator will have a windspeed limit in their operations manual, which may depend on the pilot's experience as well as the type of aircraft.

@DanHulme You mean different limits are tabulated for different experiences of pilots? A fleet would have a wide variety of pilot experiences. How does an Ops Manual mandate all this?

@curious_cat The Ops Manuals at the 747 carriers I flew for in the 1990s provided for different experience levels by specifying that for certain circumstances, the captain had to be flying the airplane. Further, insofar as required weather minmums, Captains with less than 100 hours as captain had higher minimums. However, no differentiation was specified for crosswinds. FWIW, in 747 simulators, we would occasionally crank the xwind up to 40-45 knots, and land without problems. I never made an actual landing in that much of a xwind.

The maxima of wind gusts which need to be considered in the structural design of aircraft to achieve certification are up- or downdrafts.
FAR part 25.341 says:

The following reference gust velocities apply:  (i) At airplane speeds between V$_B$ and V$_C$: Positive and negative gusts with reference
  gust velocities of 56.0 ft/sec EAS must be considered at sea level.
  The reference gust velocity may be reduced linearly from 56.0 ft/sec
  EAS at sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust
  velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000
  feet to 26.0 ft/sec EAS at 50000 feet.

Since wind rarely flows into or out of the ground, vertical gust speeds are less of a concern during takeoff and landing. Here, lateral speeds (crosswind) and changes in horizontal speed (gradients due to microbursts) constitute the most important limits.
For crosswind limits, FAR part 25.237 states:

(a) For land planes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, demonstrated to be
  safe for takeoff and landing, must be established for dry runways and
  must be at least 20 knots or 0.2 V$_{SR0}$, whichever is greater,
  except that it need not exceed 25 knots.

In the end, it is up to the pilot to decide whether to attempt to land in adverse conditions, to go around, or to fly to an alternate destination. This answer gives some criteria which can be used by pilots to decide if they better go around. Airports are closed rarely and only when severe weather approaches, such as thunderstorms and tornados, but then it is not due to a specific wind speed, but to the expected maximum speeds, which then will far exceed certification limits.
Vertical gusts become more stressing with speed, because at the low lift coefficient of high speed flight a vertical gust will produce a relatively higher increase in lift coefficient than at low speed. In extreme cases, the wing will stall due to the gust, which will limit the additional structural loads.
Horizontal speed gradients, which are caused by downdrafts hitting the ground and spreading out, are most harmful at low speed. First the aircraft will encounter a headwind, which slows it down, and then rapidly cross into a tailwind which will cause airspeed to drop, in extreme cases below stall speed. This can even crash a large airliner.


text:
Where should I sit to experience the least turbulence?
Following on from Why are first/business class seats at the front?, in which part of a large commercial jet cabin would I experience the least turbulence? On the one hand I would have said that sitting near the wings (near the centre of gravity) is best, but subjectively it feels like the ride near the front is "smoother". Can anyone give me some more solid information? I imagine it also depends on the aircraft type?

The answer is in the post you linked to.  Therefore, this is a duplicate.  Not sure what @Iceman is doing ;)

I think the answer says turbulence in front is less than the back, not it is minimum there.

I've voted to re-open this, as although the linked post does have some reference to the front generally being smoother, it does not fully answer the question being posited here, nor is the question a duplicate of the one posted. Although the reference in the other answer is correct, it is not necessarily a complete answer. For example it may be that "Business class is at the front because the front is smoother than the back", when splitting the cabin into exactly two parts, but it may actually be that there is a more specific answer that is relevant if you are choosing one seat for one purpose

On the ground .

The elasticity of the fuselage does indeed dampen the load factors from gusts somewhat. Therefore, gust-induced accelerations are a little higher over the wing than in the forward or rear fuselage. If the gust causes a pitching motion, this creates its own accelerations which adds to the bumps from the vertical accelerations. Elevator deflections cause similar pitching motions, and their accelerations are more pronounced in the rear fuselage since the aircraft will pivot around a point ahead of the center of gravity.
The main factor, however, is the location of the axis of rotation of the Dutch roll eigenmotion of airliners. This is a weakly damped oscillation which creates lateral accelerations which are bigger the more aft you sit in the fuselage. The Convair 880 was hurriedly test-flown with nobody sitting in the back, and the pilots were happy with its comfort level. The first passengers, however, got seasick from the Dutch roll motion, which was only detected when the passengers sitting in the rear of the cabin complained.
Modern airliners offer better comfort thanks to artificially enhanced stability, but even there you are better off sitting in front. I once flew on an Egypt Air Boeing 777 from Cairo to Frankfurt, and the captain flew through the tops of Cumulus clouds sitting over the Alps. Soon the obvious sounds and smell told me that people in the back were filling their air sickness bags. Maybe half of the rear third of the seats was affected, while the forward two thirds had only a few cases of airsick passengers.

The seats over the wings should have the least turbulence as they are near to the center of mass and center of pressure(over which the lift acts).
The front and rear should experience more (with slightly more in the aft).
Thought I'm not able to find any scientific studies into this, see here and here.

authors: ياسر عبدالقادر

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Can a barrel roll really be executed at a constant 1G?
I see this stated from time to time, for example in an answer to Can large airliners do aerobatics such as loops?

A barrel roll is one thing; they can be performed at constant load factor of 1 G (assuming sufficient control authority).

In a properly executed barrel roll, however, I do not think there is any point in the trajectory where the vertical velocity is constant. In particular, the entry and exit of the maneuver require transitions from level to ascending, and from descending to level trajectories, respectively. In these two cases, at least, I cannot see how a load factor greater than 1 can be avoided.
This IAC guide states a maximum in the range 2.5 - 3 (and a minimum of about 0.5), though it is probably not intended to be a minimum feasible value.

+1 for thinking while reading and bringing this to our attention.

Mandatory Bob Hoover ice tea video:  https://www.youtube.com/watch?v=V9pvG_ZSnCc

Clearly, when the nose is below the horizon and rising, the G-load must be greater than one. But it doesn't have to be much greater than one.

@CharlesBretana I think you have a point, though there is also the issue of maintaining 1G while inverted yet still keeping the axis of the entire maneuver horizontal. Perhaps a pear-shaped (narrow at the top) barrel would allow that, and in the video of Tex Johnston rolling the Boeing -80, that appears to be what he does, though it is difficult to tell whether the camera is panning.

@sdenham,  Actually, I misspoke slightly in my earlier comment, you can remove the phrase " "nose is below the horizon' Any time the nose is rising, (or to be perfectly accurate, when the flight path angle is increasing)  the vertical component of lift must be greater than the aircraft weight, which is impossible if aircraft G is less than or equal to one.  F=ma, and if flight path angle is increasing, vertical velocity is increasing, so vertical acceleration is positive. Can't have acceleration without force, so total vertical Force must be positive i.e., greater than aircraft weight.

You are absolutely right, a load factor of greater than 1 is impossible to avoid in a proper barrel roll. The barrel part of its name comes from the spiral path the aircraft needs to perform in order to add a centrifugal acceleration which is greater than gravitational acceleration at the top of the roll. This is the condition to ensure a still positive normal load factor in the reference system of the aircraft through the whole roll. Therefore, at the bottom of the roll the normal acceleration needs to be greater than 2 g.
I took the liberty to edit the wrong answer. Given that it has received 14 upvotes at this time, it should teach everyone around here a lesson how much to rely on the number of votes to assess the correctness of an answer ;-).

I see that you have not repeated my mistake of assigning a unit to a dimensionless ratio.

You say "proper" barrel roll. Well then, how about improper? I.e. counteract (or enhance, when overhead) all centrifugal forces by adequate acceleration, effectively (I think) creating a pretty spiral-like, curved trajectory. Or does that not count as one?

@PeterA.Schneider: Perhaps as opposed to a simple roll,
aka aileron roll.  That wikipedia article has a diagram comparing various kinds of rolls.  Thanks to the "do a barrel roll" meme from Starfox, many people don't know what a real barrel roll is.  In the Starfox games, you can only do an aileron roll, even though the game calls it a barrel roll, and the meme gets applied to any kind of rotation, which of course mostly aren't barrel rolls.

The claim is true in theory (spare some mathematical nitpicking), but false in practice. The mathematically sound version of the claim is:

A barrel roll can be performed at a constant load factor of 1+ϵ, where ϵ is an arbitrarily small number greater than zero.

For easier analysis, we can reduce a barrel roll to a recovery from a dive:

Start in straight and level flight. Use the elevator to maintain a load factor of one at all times.
Push the stick hard over and perform an aileron roll while maintaining a load factor of one.
During the roll, the vertical component of the lift vector will be less than gravity (sometimes even negative). The aircraft inevietably starts to descend.
Once you brought your wings back to level attitude, you will find yourself in a dive, still maintaining a load factor of one.
Recover from the dive.

Two methods to reduce the required acceleration become immediately obvious:

Recover slowly, employing a load factor only slightly larger than unity. In practice, the recovery must be complete before the aircraft overspeeds or impacts the ground, both of which happen rather quickly.
Perform the aileron roll quickly to minimise the time in non-level attitude. The less time you spend in the roll, the slower the resulting descent will be. If you can perform the roll in zero time, there will be no dive to recover from. In practice, the aircraft's roll rate is limited.

I think you have made a good case for the feasibility of making some sort of roll maneuver at a load factor close to 1 throughout, I am not so sure it could be called a barrel roll, at least by the IAC definition.Maybe arguably a degenerate form of a barrel roll in the limit as the barrel radius goes to zero and the roll rate goes to infinity, but why not call that an aileron roll?

@sdenham - A degenerate barrel roll can look like either a loop (with some slight sideways offset) or like an aileron roll (with some slight corkscrewing), but it is not identical to these as the instantaneous accelerations and roll rates are different. Nevertheless, the key to a real world low-g barrel roll is to perform it very much like an aileron roll and not like a loop.

If you do, it would look sloppy as the idea of a barrel roll is the aircraft rotates at a constant radius about a point off one of the wings, creating a helical or spiral flightpath.  The erect portion of the maneuver would require around a 2g load factor to perform correctly, the inverted section reducing to 1g or so.

In a classic moment of airliner history, Tex Johnson barrel rolled the Boeing Dash 80 (the 707 prototype) over a crowd of potential customers. Not once, but twice. And this was the Dash 80's first flight in front of the public.
Johnson reported that he kept the plane at 1 g throughout the maneuver, thus posing no unexpected stresses on it. True that it probably wasn't of competitive aerobatic quality...
but, true to Johnson's intent ("selling airplanes") Boeing did end up selling a lot of 707's.

Could  you expand on the physics of how the entry and exit of that maneuver could have been accomplished at 1G? While it may not have been a textbook barrel roll, it has all the essential features, including the entry and exit accelerations I mention in the question.

You can cancel gravity during the barrel roll by superimposing it on a parabolic arc.  Sometime before and/or after the roll you will have to exceed 1G to transition from/to level flight.

The question was can an airliner execute a barrel roll at 1G. I believe Tex Johnson answered that question in the most practical way, by doing it. Note the photo in the linked article, with the dash 80 inverted over Seattle. How is that not an answer to the question?

@aml That is an ingenious solution, though I think it is pretty clear from the video that this is not the trajectory that Mr. Johnston took the 367-80 on. On the other hand, I think it is quite possible that in his comment, he was tacitly excluding the entry and exit phases.

No, Tex Johnson performed an aileron roll, not a barrel roll.

Really? Tex Johnson, test pilot extraordinare, Bill Allen (Boeing's president who really flipped when he saw that), and just about every other aviation person who witnessed this called it a barrel roll. That seems like rather expert opinion.

No, he just did a conventional aileron roll.  Best guarantee that the maneuver will be executed as a 1g maneuver throughout.  Even Johnson himself referred to it in this manner.  A lot of people will confuse barrel rolls with aileron rolls; they are separate maneuvers.


text:
What runway length and width is needed to land a G650 and take off?
How long and how wide should the runway be to take off or land a Gulfstream 650 on private ranch property?

It should be noted that you will need some clear space on both ends of the runway as well, your over all land needs will be notably longer than your book runway distance.

At what altitude?

Depends on altitude, air temperature, weight, winds, and obstructions

This one is based upon aircraft loadout, field elevation, surface winds and ambient atmospherics.  You will have to consult the airplane flight manual and performance tables specifically approved by Gulfstream for use in the particular G650 in question for required runway length.  Typical takeoff lengths are therefore going to vary between about 3000 - 8000 feet depending on the variables listed above with a quoted metric at 5858 feet at MSL, calm winds at STP and MTOW.
Often times, the distance to clear a 50 ft obstacle at a particular airfield is more useful to the flight crew than the mere runway length needed to take off; these figures are also published in the performance tables for the jet.
As to runway width, I'm not aware of a specific number for this; at a guess at least 3x the wheel track of the main gear and at least, say, 100ft of clearance for obstacles from centerline.  That's still a pretty tight squeeze and unrealistic for normal operations.  Given the kinds of airports this jet will likely be operating from, this really isn't concern.

Unless you want your Gulf Stream to look something like this, I would be more concerned about the strength of the runway surface in relation to the ACN of the G650 (which does not appear to be published). BTW, this is a G450 in the picture.


text:
Flying to Paro, Bhutan requires a special crew, what are the regulations?
I would like to fly from Geneva to Paro, Bhutan on Gulfstream GIV-SP. I was informed that I have to take a specialized pilot from Delhi. Can anyone guide me what are the formalities and what are the solutions?

Where are you flying from? What makes you think you need a specialised pilot?

Flying from Geneva. Contacted BCAA and that is what they informed.

@J.Hougaard Looks like only a few are authorised to fly there- see Why is a special certification needed to land at Paro Airport?

Yes we saw that. That comment also support what BCAA informed us. BCAA also informed that we can get specialised pilot from Delhi. But did not gave the address or contact at Delhi. So I put the quary. Still someone can help in finding the Delhi address or contact.

@ChatterjeeS I suggest that you take the airline route unless that is absolutely not possible. In addition to the crew, you also have to consider whether the GIV is permitted to fly there- only a few aircraft have been operated there and the pilot should be certified in that.

You might get an answer here, but the charter company or aircraft operator should be able to find out. They have to actually plan and operate the flight, after all. There are specialized companies that provide logistics and local handling services all over the world for private flights, and the jet operator probably uses one of them already.

I believe "you have to take a specialized pilot from Delhi" just means that you'll need a pilot who is current for Paro for the leg from Delhi to Paro, and implies nothing about whether such pilot is available in Delhi. There might not even be a pilot current for Paro and rated for GIV at all—whoever gave you the message probably simply didn't know whether there is and where to find them.


text:
Feasibility of using secondary props to generate power on an electric model aircraft
Beginning with the aeroplane itself, it is a small, fixed wing model aeroplane, reminiscent design-wise to a Cessna 172. It is completely battery powered and has a primary prop attached to a motor housed in the nose of the aeroplane.
I was wondering how feasible it would be to attach two props behind the wings of the aircraft, one on each wing, connected to a generator which would then connect to the main battery, so as to recharge the battery while the aircraft is airborne.
I haven't been able to find much information regarding this, so any ideas would be greatly appreciated.

It seems to me to be somewhat pointless - your extra 2 props will create more drag, thus the main drive motor will need to work harder for the same result as without them. They're effectively cancelling each other out, and the laws of thermodynamics tell us that energy will be lost from the overall system.
It's much like connecting a solar panel to a lightbulb; physics just doesn't allow for a 100% efficient system, so it can't work.

Ah yes, the  perpetual motion machine. Unfortunately the second law of thermodynamics prevents that from happening.


text:
Which 'pusher' propeller aircraft have been used in active military service?
The only 'pusher' aircraft I know of that has been used in active military service is the SAAB 21. There are several prototypes out there, but I can't see that any others were used.
Which, if any, other aircraft are there? Maybe the Japanese 'shinden' but if memory serves me that is only a prototype.

The Cessna Skymaster was/is used by some militaries.

If you also count drones, the General Atomics MQ-9 Reaper Drone is a pusher propeller aircraft in active service.
Here are some others:

Convair B-36 Peacemaker
Vickers F.B.5 "Gun Bus"
Airco DH.2
Curtis Model E
Royal Aircraft Factory F.E.2
Royal Aircraft Factory F.E.8
Voisin III Bomber
Boeing GA-1

You may notice that a lot of these aircraft are very old designs. The problem is in 1914 the US Army (there was no US Air Force yet) banned pusher type aircraft after a number of pilots died in them. This mentality carried over on the other side "of the pond" and very few pusher type aircraft entered service after 1916. The SAAB 21 was responsible for the development of ejection seats because a major problem with bailing out was hitting the propeller. Because the prop was behind the pilot, bailing out meant getting pushed through the wood chipper...
The B-36 is a good example of this configuration in its limits. It is a massive aircraft with a 19 foot propeller (second largest propellers in use). Engine fires were common due to mounting the engines backwards and not getting sufficient heat over the carburetors. The aircraft was later modified as the XB-60 with swept wings and 8 jet engines but was inferior to the Boeing XB-52 (later becoming the B-52) and was abandoned.
The other problem is that a pusher type design contributes a lot to the drag of the aircraft and is relatively inefficient compared to tractor designs. By the time swept wings were in fashion (mostly because jet engines pushed speeds higher and higher),  the need for propeller driven military aircraft dropped dramatically in the jet age.

There's also the Cessna O-2 Skymaster, which has been used as recently as 2010

@SteveKuo I mentioned that in the comments below the original question, although I narrowed my answer to just pusher style aircraft, not pusher/tractor aircraft.

Good answer! The early years saw many pushers, and the Canadian Aviation Corps even was 100% pusher equipped, owning one single Burgess-Dunne pusher in its brief life.

… except the last paragraph. Pusher design is more efficient. The disadvantages are elsewhere.

The Cessna O-2 (337) quickly comes to mind.
Then there are various UAVs, the MQ-1 and MQ-9 are the most prominent.
And the Convair B-36 is another good one.
The Dornier Do. 335 heavy fighter.
The Kyushu J7W1.

The German Do335 'Pfeil' was a twin-engined fighter of the forties. One engine driving a tractor prop at the front, and the other driving a pusher prop at the tail, with a long transmission shaft.

Here are some more pusher types to add to the list. I tried to ensure that all had some kind of active service record, not including evaluation prototypes or designs not built.
With a couple of exceptions noted below, all of these served in the First World War.
Fighters and bombers with single pusher propellers:
Farman MF.7
and MF.11;
Breguet Bre.4
and Bre.5
AGO C.II reconnaissance aircraft, single pusher propeller.
Various German bombers, each with two pusher propellers:
Friedrichshafen G.II and G.III;
Gotha G.II,
G.III,
G.IV, and
G.V;
Rumpler G.I, G.II and G.III;
and Albatross G.III.
Flying boats with single pusher propeller:
AD Flying Boat;
Aeromarine 40;
Curtiss Model F;
Hansa-Brandenburg CC
and W.18;
Lohner L;
Macchi M.3
and M.5;
Grigorovich M-5
and M-9;
Beriev MBR-2; and
Supermarine Walrus.
(The last two were operational in the Second World War.)
Grigorovich M-16 reconnaissance floatplane, single pusher propeller.
There were also a number of aircraft that had both tractor and pusher propellers, such as the
Caproni Ca.4 bomber (two tractor propellers and one pusher propeller) and the
Zeppelin-Staaken R.VI bomber (two tractor propellers and two pusher propellers).


text:
Are there any recreational gliders with a yoke?
All gliders / sailplanes seem to use a single central stick to manipulate the primary control surfaces. In other airplane types, we also find yokes and sidesticks. Are there any glider designs, historic or modern, which do not have a central stick but instead use a yoke, sidestick, ...? Why (not)?
For the purpose of this question, Touring Motor Gliders (TMGs) and cargo gliders are out of scope.

The Gimli glider had one ;-)

Does this describe a glider?  "From the linked page above, "On March 5, 1950, Bob was flying his P-38 and took advantage of this little understood weather phenomenon to try something new. He soared the P-38 with both engines dead and propellers feathered for more than an hour between 13,000 feet and 31,000 feet. Maximum climb rate was 3000 feet per minute.""  Source -- https://www.aerosente.com/2009/04/the-sierra-wave-project.html -- also widely described elsewhere.  ; )

Would any of the Horten gliders qualify? None of them had a central stick, all used a side-mounted stick.

@PeterKämpf Yes, feel free to add your answer ;)

@Jim Arguably the Gimli glider was just a large TMG...

@quietflyer Done

Are there any glider designs, historic or modern, which do not have a
central stick but instead use a yoke, sidestick, ...?

The Schreder HP-18 sailplane, offered as a kit to homebuilders, was designed with a sidestick.  I've seen one "in the flesh".  Some have now been converted to a central stick, and some were originally built that way.
One can easily see why a sidestick could be an advantage in a sailplane with a very slender cockpit, with the pilot seated in a prone position-- there would be less of an issue with the pilot's legs limiting the stick travel-- but there are some ergonomic disadvantages as well.
The Applebay Zuni sailplane also featured a sidestick.  In this glider, the sidestick's fore-and-aft-motion was accomplished by sliding rather than by pivoting (the same may have been true of the HP-18's sidestick), and
the "breakout" force or static friction that needed to be overcome to move the stick in the pitch direction was rather high.  This made it more difficult to control the glider at high airspeed-- the pilot could accidentally impose a high G-load when intending to only make a small input--  which may have contributed to a fatal accident.1
"Shifting gears" a bit, ultralight sailplanes that are designed to be foot launchable are usually equipped with sidesticks.  The necessity for the pilots legs to be able to hang out the bottom of the aircraft makes a sidestick much more logical choice than a center stick.  Many of these aircraft may also be aerotowed, utilizing wheels rather than the pilots legs, and thus are functionally equivalent to more conventional sailplanes.  Motorized self-launching versions of many these aircraft also exist (again utilizing wheels rather than the pilot's legs for landing gear), and retain the sidestick configuration.  Examples of foot-launchable ultralight sailplanes include the Aerianne Swift and its Bright Star predecessors, the Bright Star Millenium, and the Ruppert Archaeopteryx.
Another class of gliders that usually have sidesticks are those where the pilot flies from a prone position.  Historical examples include the wings of the Horten brothers, several of which were flown in soaring competitions.
A modern example that would fit the description of a "recreational glider" is the ATOS "Cage" ultralight sailplane.  The "Cage" evolved from the line of ATOS rigid-wing hang gliders in which the pilot is suspended prone below the wing with his hands on a triangular "control frame".  The "Cage" uses wings from the "ATOS" hang glider series, but moves the pilot up into an enclosed fuselage between the wings, while retaining the prone position.
As for control yokes-- in modern high-performance sailplanes, where the pilot is seated  in a semi-recumbent position in a slender fuselage, space is at a premium.  A control yoke takes up more space than a control stick, and so a control stick is the better design choice.
Footnotes:

Source-- anecdotal information from long-time sailplane pilot familiar with the incident.  The accident is is also mentioned-- with no cause offered-- in the last link below.

Links:
Article on Schreder HP-18 in National Soaring Museum collection (mentions side-mounted control stick)
Wikipedia entry on Schreder HP-18 sailplane (mentions side-mounted control stick)
Link to a center-stick conversion kit for Schreder HP-18 sailplane
Link to an interesting article about a highly modified HP-18
Wikipedia entry on Applebay Zuni and Zuni 2 sailplanes (mentions side-mounted control stick on Zuni, and notes that it was replaced with a center stick on the Zuni 2 "to remove unwanted roll during high g manoeuvres".
Page on Applebay Zuni and Zuni 2 sailplanes from on-line tour of National Air and Space Museum Garber facility
Wikipedia entry on Aerianne Swift foot-launchable ultralight sailplane
Webpage for Aerianne company (navigate to "Swift Aircrafts")
"Hang Gliding Wings Bible" entry for Bright Star Millenium foot-launchable ultralight sailplane
Webpage for Ruppert Archaeopteryx foot-launchable ultralight sailplane
Wikipedia entry on the Horten brothers
Link to discussion forum (in German) on ATOS "Cage" ultralight sailplane.  Some photos are included.

It would be interesting to know why the sidestick controller used in the F-16 does not cause "unwanted roll during high g manoeuvres", and whether or not it would be practical to adopt the same ergonomics to a sailplane.  Perhaps a sailplane cockpit does not have room for an armrest that is used in the F-16 to stabilize the pilot's forearm?

Well, first, determine what it was about the Zuni sidestick that did cause / induce / contribute to an increased risk of those unwanted roll inputs. I'd suspect that the long travel necessary for a mechanical connection to the elevator makes an armrest more problematic, since the forearm isn't in a single position. The F-16's sidestick, by contrast, is a computer input, and has nearly zero travel, meaning that the forearm doesn't appreciably move and can remain consistently supported in all regimes.

Ironic for me to come across this question. I own an HP-18 glider and yes it has a side stick control.  The Diana 2 glider also has a side stick.  I have researched and have yet not come across any other gliders with a side stick.
.
Perhaps a few things to say about the HP-18. I bought my glider with a centre stick conversion kit and I had intended to install the centre stick.  I found that I also wanted to adjust the instrument panel and bring it closer to me (the instrument panel is located below my knees and I sit almost lieing down so I can't reach the instrument panel).  The centre stick conversion kit would prohibit me from moving the panel closer. So I decided not to install the centre stick conversion.
.
The HP-18's side stick handle is actually located higher than your shoulder meaning the angle from your elbow to your hand is upward implying your blood has to travel upward towards your hand - this will become tiring.
.
I was inspired by the Diana 2 glider that has the control stick at your waist.  This is far more ergonomic.
.
I found that I was able to move the HP-18 control stick to my waist area - the glider is actually wide enough to give me comfortable space with the stick alongside.  I did have to modify and decrease the 'aileron travel' meaning slight movements of my hand from left to right will result in large aileron movements.  The elevator movement is still heavy though.  I have checked and double checked that my conversion has nothing to do with the heavy elevator.  I am convinced that the HP-18 V-tale is the reason for such a heavy elevator.
.
So far my conversion feels really cool - I hope I don't kill myself flying this glider.

Welcome to Aviation SE. Is it possible to include a photo of the cockpit layout, this may give more context both to your answer and in fact the question itself?

The Waco CG-4 first flew in 1942 and was selected to be the standard USAAF assault glider.

It had a steering wheel.
Does that count?

 Picture credit: flight-manuals-online.com

The question does exclude cargo gliders, certainly different space constraints here than a sport glider!

Sorry, cargo gliders not welcome...

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Is a glider on a winch directionally stable?
Let's say I were to fly a glider with the stick and rudder exactly centered laterally, while making the appropriate pitch inputs, during a winch launch.
Would the glider remain wings level throughout the launch or will it diverge to one side? My intuition says it will be stable, because on the flip side you need to apply constant stick and rudder inputs to steer into a crosswind during winch launch. Then again, that is for coordinated flight, so perhaps stability suffers if we allow a sideslip to develop. Also, I've only flown a very gentle ASK21 trainer, perhaps it's different for less stable aircraft.
I'm looking for an answer that relates the forces acting on the airframe to a stability argument: so both the forces and moments due to the winch, as well as the sideslip / roll coupled forces. Free body diagrams and/or equations of motion are highly encouraged.

Related question: is a hang glider directionally stable during winch launch?  Answer: far from it!

Someone ought to give this question a maximum-possible bonus-- only to be awarded to answers involving actual in-flight testing!!!. Gut feeling- no it will not be directionally stable.

@quietflyer Maybe on a model glider. I won't reward messing about during the most dangerous part of sailplane flying...

It may be relevant to research -- "freeflight towline gliders model airplanes"

Having been on both ends of the tow for hang gliders and paragliders I can assure the OP that these aircraft love to ‘lock out’ in yaw/roll and hang gliders in pitch.  Uncharacteristically this is quite spontaneous and the ‘pink dangly thing’ (i.e. pilot, for the avoidance of doubt) is not generally to blame.

@Frog - your comment could be a "springboard" for a new question- what makes "dope-on-a-rope" parasails so directionally stable on tow, and why are hang gliders so radically different in terms of stability on tow, and where (and why) do paragliders fall on this continuum?

@quiet flyer - I think I know the answer to that but no spoilers (see what I did there?) :-)

If you want the short answer, skip to the bottom.
If we look at a side-view of a winch launch, we can get a sense of the forces on the glider during launch.  The below is a representative example taken from a numerical study of a winch launch, including a dynamic model of the aircraft, cable, and winch:

Looking at the angle of the cable with respect to the aircraft, we can infer that the cable, while providing a non-negligible downward force (relative to the aircraft), it is primarily providing forward velocity to the aircraft.  Per the study (and experience flying a winch), the primary job the pilot is to maintain a desirable dynamic pressure (read: airspeed), which we do via managing pitch angle.  This is verified if we look at our experience as well as results from the paper (copied below).  Please excuse the first few seconds of the experiment as it doesn't include ground roll, but note how the pitch angle, $\theta$, and the climb angle, $\gamma$, trend with glider's true airspeed, $V_a$ (not to be confused with the published maneuvering speed for the aircraft...I can't help their notation, sorry).  In short, the winch is giving energy to the aircraft, which is then converted into altitude via management of the glider's pitch angle.

All this to say: a winch launch doesn't naturally produce lateral or directional moments.  The aircraft is receiving energy from an outside force, but the cable giving that force is nominally attached at the aircraft's CG and is allowed to pivot freely.  In other words, it is not contributing to the stability of the aircraft save through its influence on the aircraft's speed and angle of attack.  This does end up producing an opportunity for the pilot to excite the aircraft's phugoid mode (a pitching response) while pitching up into the climb, but that isn't going to generate a yawing/rolling motion.  While on the ground, the rope can produce highly de-stabilizing yaw moments (should a wingtip catch the ground), but, once in the air, the winch cable isn't intuitively stabilizing or destabilizing the aircraft (as opposed to an aerotow cable, which provides strong directional or pitching moments due to where it is attached).  Hence, if all is perfect we can consider a glider on a winch launch as a glider in the air, albeit with highly loaded wings.  And, if everything is perfect, that is all it is.
However, let us consider what isn't perfect.
THE SHORT ANSWER
A glider is designed to be laterally stable -- if only because their long, slender wings flex upward as they are loaded (thus giving the glider some more dihedral) -- and the extra energy given by the winch lets us load the wings more than normal as we pull back during a winch launch to gain altitude.  Lateral stability is typically expressed as a rolling moment that corrects for sideslip.  If the aircraft is maintained in perfectly coordinated flight through the initial phases of launch (i.e., where it is generating airspeed and safety margin above stall speed), the aircraft will not experience sideslip and will not roll.  However, the key assumption is that the aircraft's ground track is parallel with the winch cable.
The winch will continue to pull the glider toward itself throughout the launch, regardless of the yaw angle of the glider.  As mentioned before, this is particularly problematic on the ground (as it can produce an aggressive ground loop), but, in the air, this results in an increased sideslip angle instead.  A laterally stable glider's natural response to a sideslip is to roll opposite the direction of the slip to correct for it.  Hence, if the ground track of the glider is not parallel to the direction of the cable pulling on the aircraft, the winch cable will give the glider some sideslip.  The aircraft's response will be roll, and it will be up to the pilot to maintain a safe attitude.
Long story short: don't let go of the stick and rudder during a winch launch.

Never done a winch launch, only aerotow.  Looks like fun though. If my club got a winch that could get you to 1500 ft for 15 dollars, instead of a $55 aerotow to 2000 ft, ppl would be lining up for the winch most days.

Plans are now available for a human-free computer-controlled winch for launching gliders--

Gas engine or electric?

Of course you'd need a big battery, or a really big 240 v extension cord lol.

I'm sorry but I'm not convinced by this answer. The winch is not attached in the center of mass but below it, so there's definitely some roll torque. The real question is what happens after a roll is introduced due to sideslip. Will the roll angle keep increasing or stabilize?

@Sanchises - if I read you correctly, you want me to keep going with the answer: my answer presently takes you up until you get a sideslip-induced rolling moment, but you want the dynamics after that?  I would think it would be an uncoordinated turn, since no rudder has been applied, the wing pointing forward would be at a negative AoA, and the opposite wing has a positive AoA.  In short, it seems like you would see some sort of fishtailing motion as the yaw would lag behind the roll.  Loads could exceed the flight envelope in those turns, since the winch is providing consistent energy input.

Exactly that! Stability is all about what happens after the initial perturbation. If you think it will be a fishtailing motion, it then becomes a question of damping....

The winch cable adds a force which acts in the vertical plane of symmetry at a point below and a bit ahead of the center of gravity. In zero crosswind there is no lateral force component added by the cable and lateral stability is not affected. A laterally stable glider will still be laterally stable during the winch launch.
This is not to say that longitudinal stability is not affected: The cable adds a pitch-up moment in the early stages of the launch and a pitch-down moment in the later stages. Both cases need to be considered when sizing the horizontal tail during design. All-flying tail designs are known for the risk of pitching up violently before the tail can become effective (because of high deflection and low speed). In a Cirrus or LS-1 always keep the stick centered during lift-off to ensure that the tail can do its job from the start!
In a crosswind the winch cable force is still acting in the plane of symmetry but the glider will drift sideways if no corrective action is taken. This is equivalent to flying a kite - eventually the glider will be on a different course determined by the direction of the wind. The glider's lateral stability will turn it into the wind while the wind pushes it sideways until the crosswind is reduced to zero.
Only when the glider pilot corrects for the crosswind will lateral cable forces emerge. By applying rudder during acceleration (aileron deflections only to keep the wings level!), the glider will fly at an angle to the winch cable during lift-off which adds a lateral force below the center of gravity. Now we have both a side force and a rolling moment added by the cable. If no further action is taken by the pilot, the airplane will develop a sideslip from the side force and a windward wing down rolling moment. However, depending on the initial rudder application, this added side force might

reduce the sideslip because the initial rudder overcompensated for the crosswind. Now the rolling moment from the sideslip due to overcompensation and from the cable force will add up and drop the windward wing. The continuing acceleration of the glider during the initial stage of the launch will over time reduce the sideslip angle caused by the crosswind, so this scenario is rather likely. Now the glider will roll into a bank angle where the side force of the winch cable is again in the plane of symmetry and the glider nose is pointing into the wind. Lateral stability will drive the sideslip to zero and the glider ends up in a state similar to the one at zero crosswind, only with a bank angle which will eliminate the sideways component of the winch cable. As the glider climbs higher, this bank angle will diminish and any side force from the cable will pull the glider into a sideslip which in turn will let dihedral take care of the wrong bank angle.
add to the sideslip because too little rudder was applied initially. This will now look similar to the case with no crosswind and the glider will drift sideways but slower than if no correction had been applied.

The considerate pilot will overcompensate and fly the glider to a point above but laterally offset from the winch in windward direction so the cable will not drift too far leeward during the final stage after the winch cable has been released. This makes the first scenario more likely. At all times the lateral stability of the glider will be maintained and the cable force will have little influence because this stability will drive the cable force into the plane of symmetry.

Perhaps one way to look at it would be to use the attachment point of the winch to the airframe as a thrust vector.
In a winch launch with no wind, with the attachment ahead of CG, you have an arrangement curiously like a kite.  As the aircraft speed increases and it lifts off, there will be downforce on the nose working against the pitch up tendency of the glider.  This keeps the line in tension and allows the aircraft to accelerate by lowering wing AoA.  Effectively, the thrust line is now down as well as forward.  The aircraft is stable in roll and yaw, but not free flying in pitch.
As for lateral stability in a cross-wind, it might be best not to even attempt it with a winch.  Simply turn the winch so it pulls directly into the wind.
The placement of the winch attachment under the CG would introduce a destabilizing rolling component that would work against the righting tendency of the aircraft but ...
the aircraft is accelerating in the direction of the winch pull and has it aerodynmic stability to increasingly dominate thrust line deviations.  Rolling torque forces of the "thrust line" and crosswind could be calculated against the aileron torque theoretically.  There also may be crosswind launch specifications in the POH or known among experienced pilots or instructors.
In short, find an experienced winch operator and stay in the safe part of the envelope.

The attachment of the winch always is directly under the CoM. During acceleration, there can be a very nasty nose up pitch moment. There's definitely crosswind limitations in the POH since most fields don't have the luxury of turning the winch straight into the wind.

@Sanchises I think you're ignoring the pitch contribution of the lifting wing.  This is why DLG gliders reduce trim on launch but winch launch must maintain tension.  So trim would be critical.  For cross-winds, a U-2 like wing wheel may be possible to support the downwind wing.  It seems pretty dumb not to move the winch pull direction anyways.  I might find a field that does, or ask Peter or John to tow me up!

Anyway I'm not interested in pitch stability but directional/roll stability. A winch needs a  >1km long track, so you can't just change the "pull direction" if you don't happen to have a stretch of grass in that direction.

@Sanchises I accept that, other than the (dropping) wing wheel and expert instruction there is little more to say.  But the first few feet of roll would be critical as counter-acting aerodynamic forces will be relatively small.  Since it varies with speed, what to look for in the literature might be a control "threshold".  I wouldn't feel so bad if the winch acceleration was sharp, as long as it got me away from the ground.

Acceleration is in fact very sharp precisely for this reason. Instead of a wheel, a human tip runner is employed. But I'm more interested what happens halfway the winch launch than the ground roll which, with a good winch operator, is very short anyway.

@Sanchises well, working backwards (and with minimal vertical torque arm between CG and attachment point) it would seem that the glider will have adequate roll control and more directional stability because it is accelerating.  A little like stepping on the gas in a turn can help in a car.  Bottom line:  the winch launch must be flown.  It would seem the winch acts as rudder (directional stability, pulling ahead of CG), so it would be ailerons as needed.


text:
Why am I not getting the "TO" VOR flag as I think I should?
I would appreciate some insight as to what I’m doing wrong or missing.
I just completed a XC flight practicing VOR/GPS Navigation. I was inbound to KLOL from KRNO with the lovelock VOR frequency (116.5, 040 Radial) dialed in. I was southwest of the VOR on a heading of 030 and at an altitude of 9500.
The issue is, I could not get the VOR to designate a  “TO” flag unless I turned the OBS to the reciprocal of around 220. This radial should indicate a “FROM” flag.
This is so confusing because I’ve cross-checked this on multiple simulators just to make sure I wasn’t going nuts. Dialing 220 should indicate “FROM” but it indicates “TO”.
Can anyone help me figure out what I did wrong?

Welcome to AviationStackExchange. At what altitude were you flying?

Thank you! I used to use stack exchange for programming, when I saw there was an aviation section, I knew it would be incredibly helpful.  I was flying at 9500 msl.

A quick review of the US Chart Supplement shows that the Lovelock VOR (LLC) is unusable at your current altitude and position. The VOR is 3.6 NM North by Northwest from the field. Your aircraft is 24.5 NM South by Southwest of the field. Your aircraft would have to be at or above 9600 feet MSL to receive a useable signal from the LLC VOR. This makes any reading you receive from your Nav radio receiver OBS or CDI suspect.

Source: skyvector.com

The probable reason for the VOR indication to work in a sim is that the sim does not replicate real world radio telegraphy. It is only basing the VOR CDI display on geographic position and magnetic variation.
Something of which to take note is the VOR equipment elevation of 4760 feet MSL, versus the highest point Southwest of the VOR at 6348 feet MSL. There is also a microwave tower West of the VOR. These things should be taken into consideration in flight planning.
If you were to use actual IFR charts, routes and procedures, you would notice that you are not allowed to fly below 10,000 feet MSL to still guarantee that you receive the LLC VOR signal on the Victor airway from the FMG VOR to the LLC VOR. As a matter of fact, prior to the HIXUP waypoint, the minimum enroute (IFR) altitude using GPS is 10,300 MSL. The Minimum Obstacle Clearance Altitude of 9500 feet does not guarantee signal reception.

Source: skyvector.com


text:
Why is there water coming from underneath the plane?
If you look at 2:10 in this video, you will notice water or something coming from the belly of the challenger 600. Why is this?

I'm not sure but it might be gray water from the lavatories. Unlike black water which must be stored in holding tanks, gray water can simply be dumped overboard. Hopefully someone could confirm this.

That's AC condensate, not from the crapper.

Just the plane going one more time before airborne, long flight and all... :-)

It' probably not grey water from the lavatory. You're unlikely to have somebody using the head while taxing out for takeoff. It's condensate from the air conditioning system, not unlike the water drainage you get from a car's air cconditioning evaporator on a hot day.
There are drain lines for the main water separators in the low pressure ducting, as well as drains for the ducts themselves at the lowest point in the underfloor ducting so that water can't collect in the ducts. The cabin ducting drains will use spring check valves so they are only open on the ground and close as soon as the cabin starts to pressurize. Water sloshing along the ducts as the airplane accelerates and brakes will cause the drain flow to start and stop.
In humid weather on the ground, the environmental control system's packs will be running hard in cooling mode, and will produce lots and lots of condensate water. The drains will run to the same heated mast used for the galley and lavatory.

From the Bombardier Challenger 300 Cabin Handbook:

Waste water from the galley and lavatory is drained overboard through valvesand hoses to the forward or aft drain masts. Waste water from the galley driptray and ice drawers is discharged through the forward drain mast. Waste waterfrom the lavatory sink is discharged through the aft drain mast.

Sample image of a drain mast used in a Challenger aircraft:

I believe this image of the bottom of the fuselage shows the drain mast - which corresponds to the source of the dripping water in the video you provided:


text:
Which airfoil is used on the Boeing 737 NG vertical stabilizer/rudder?
Does anyone know what airfoil Boeing uses on it's 737 NG's vertical stabilizer or the rudder itself? I can't seem to find the info anywhere and I need it for a university project.

Most probably is a proprietary airfoil but for your project a NACA 0009 or 0012 will do the job.

As stated by a professor in my university, the vertical stabilizer of the 737 uses either a NACA0012 or an exclusive airfoil designed by Boeing. Most probably, it is a variation of the NACA0012 created by them.

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.


text:
What programming languages are used for equipment onboard aircraft?
Generally speaking, What programming language is used in aviation for (ATC Radio, Radar, ILS, Auto-pilot and on-board avionics)?

Is there a standard enforced by ICAO?
Does every plane manufacturer use the programming language they like as long as it's reliable and it goes through testing?

I remember watching a documentary on YouTube last year about aviation and it said something about the EU, after WWII, started making standards for aviation systems inside Europe.
I will link the video if I can find it

see this question: http://programmers.stackexchange.com/questions/153266/what-operating-systems-are-used-in-airplanes-and-what-programming-languages-are

In keynote on Going Native 2012, Bjarne Stroustrup mentions software for F-35 is written in C++ with some limitations. Specifically he mentions exceptions are not used because it was not possible to provide hard real-time guarantee for them.

Generally speaking? This sounds a bit like a rabbit hole. Generally speaking, I'm pretty sure avionics manufacturers all do whatever they want, as long as it meets the specs.

@egid The first part of that last sentence is so true! But if they would meet the specs, I'd be only part-time employed!

@Jan, oh, that explains F-35 cost overruns :)

There are no language recommendations or requirements, other than what is sensible given the verification requirements in the various standards (e.g., DO-178B/C). For the highest design assurance levels, the generated code must be inspected down at the op-code level to ensure no known processor gotchas are invoked. You also end up having to test every part of every conditional, both the true and the false sides, and ensure that no jump instructions will take you someplace where you didn't expect it to go.
For these reasons, the big three languages in use in embedded avionics today are:

C++ (as a better C. The OO nature is often used only sparingly. Things like polymorphism are just too expensive to verify)
Ada (You may not like it, but a TON of current avionics are written in Ada, and are currently being maintained and updated with new features)
C - As close to the hardware as you want to get and still be highly productive (not intending to offend anyone who is productive in something else)

Assembler is frequently used for specific needs, but it would not be used to a greater degree than one of the above-mentioned languages.
What makes these languages ideal for avionics is that they are relatively close to the hardware and memory structure, not too abstract. Their tools are mature and well-known. People have years of experience certifying software written in them. And there are lots of programmers who can get up to speed in them quickly (Yes, even Ada).
Non-safety critical software, both on and off the aircraft, will be written in any of the big languages of the day: Java, C#, VB, C, C++, Python, Perl, Ruby, etc.
iPads are becoming useful inside the cockpit and are approved in limited situations. iOS apps are written in Objective-C, C, C++, and increasingly, Swift (Apple's new language).

Historically, FORTRAN was used a lot. Example: X-31.

SPARK is increasingly replacing Ada since it is more assurable and removes a lot of the Ada ambiguities.

A very important component of the stack is the hardware and the operating system.The hardware is usually designed to work with an operating system that runs in realtime.  Ensuring a deterministic response to interrupts by the operating system which ensures notifications are handled under one of several models of varying levels of service guarantee.
Understanding the hardware and the operating system combination (just hardware without an OS) is important in order to ensure the best choices and practices for software development.

"C - as close to the hardware as you want to get". I am always mistified by that line of reasoning, given that dealing with hardware usually needs tight timings, and C has absolutely no way to explicitly deal with time, so has to rely on hacks like volatile ordering or drop to proper assembler to count cycles, plus pray away any risky compiler optimization...

As far as I know, there is no directive on which language to use. You have guidelines on how to test and certify software, but as far as these guidelines are concerned, no language is preferred, it is a design choice.
As of today, in my (limited) experience as an engineer/programmer, I have mostly seen that non object oriented languages are preferred, since they reduce the amount of testing required. Usual arguments I have encountered primarily concern explicit memory management in often resource-limited environments.

All true, but Ada is highly prevalent in existing avionics because it was designed with verifiability in mind. New development is shifting away from this though.

Worth mentioning that not only your language choice is an issue, but so is your compiler.  Frequently, compilers have known bugs, and you must be careful to design to avoid or work around these issues.

@dvnrrs main reason Ada is prevalent is historical, not current. As those old systems (and the US military's requirement to use it, it's their baby) disappear, Ada disappears.

non-OO reduces testing? There's a first... OO was in part introduced to improve testability of code.

@jwenting given the guidelines linked in the answer, yes.

@dvnrrs, ADA was my first thought, as well, especially for military purposes. I see from the ensuing discussion that it may not be as prevalent any more.

Non oo reduce the test when you have to inspect the assembly code. Oo assembly is much harder to decipher.

This explains everything about the Boeing 777's computer systems: http://archive.adaic.com/projects/atwork/boeing.html

Honeywell was to develop the cockpit's primary flight controls in two
  projects, the Boeing 777's Airplane Information Management System and
  its Air Data/Inertial Reference System. For these projects, Honeywell
  purchased DDC-I, Inc.'s Ada Compiler System, using it as the front-end
  source for Honeywell's symbolic debugger. The two companies worked
  together for a year and a half to build the compiler's final debugger
  and the entire back-end, targeted to an Advanced Micro Devices (AMD)
  29050 microprocessor. According to a recent telephone interview with
  Jeff Greeson, Honeywell's project leader for the 777 project's
  engineering, the companies "were able to build into the compiler a lot
  of optimization features specific to our hardware."
Hydro-Aire
  selected Alsys' Ada software development tools for the brake control
  system project. The supplier used AdaWorld cross compilers with the
  Smart Executive and Certification package to ensure meeting real-time
  and FAA requirements. The compilers are hosted on Hewlett-Packard HP
  9000/300 platforms; they targeted the Motorola 58333 microcontroller,
  making Hydro-Aire one of the first companies to use the new chip.
Each
  777's brake control system includes two Motorola microcontrollers
  programmed entirely in Ada. Harry Hansen, Hydro-Aire's Manager of
  Software Engineering reported that "We find Ada an excellent language
  for the development of real-time applications." The processors control
  the built-in test (BIT) and auto-brake functions. The BIT includes
  both an on-line interface to the central maintenance computer and
  off-line maintenance capability. The auto-brake applies the correct
  amount of brake pressure during landings and applies the maximum
  amount of pressure -- without causing a tire blow-out -- during
  aborted take-offs. Additionally, the system includes hardware and
  software to prevent skids, sensors and transducers to external
  systems, and hydraulic valves.
Sundstrand, too, chose a compiler from
  Alsys, Inc. (now Thomson Software Products, Inc.). Running on a PC
  host, it generated code targeted to an Intel 80186 microprocessor. The
  Certifiable Small Ada Run Time (CSMART) executive code that interfaces
  with the language resides inside the run-time controller and,
  therefore, had to be tested and verified. It was a major undertaking,
  but not a long-term inconvenience. "Ada continues to be our baseline
  language for future electrical systems," Teske said, "for reasons of
  cost and efficiency. We are now able to reuse code. We pull out
  certain chunks of airplane software and put them into new projects."
In a recent telephone interview, senior software engineer Malkit Rai,
  who led the effort on the Sundstrand 777 electrical power project,
  agreed on the importance of Ada's support for reuse. Ada has
  permanently replaced the shop's previous high-level language, PLM,
  which was developed by Intel and is based on PL/I. "Ten to 15 percent
  of the 777 Main Channel Electrical Power Generating System is already
  in reuse," he said. Two new projects, for the Gulfstream V business
  jet and the Comanche helicopter, were able to integrate Sundstrand's
  library of common generic packages written in Ada for the 777.
In
  fact, the Sundstrand power systems' 80,000 lines of code were in
  themselves reused by 10 to 15 percent. The embedded software's small
  size proves that Ada is well-suited for projects under 100,000 lines
  of code, as well as for large efforts. The 777's Cabin Management
  System, for example, is a communications module mounted on the 777's
  back seats and offers passengers a variety of services and is only
  70,000 lines.
Putting Together a New Architecture
In comparison, Honeywell's Airplane Information Management System
  (AIMS) project consists of the largest central computer on the
  jetliner; it runs 613,000 new lines of code (defined as body
  semicolons), taking up 15,656 kilobytes (KB) of disk space and 4,854
  KB of random-access memory (RAM). With redundancy, the software runs
  to 46,191 KB and 10,732 KB of RAM. A multiprocessor, rack-mounted
  system, the AIMS replaced many of the line-replaceable units and
  reduced hardware and software redundancy.
Two AIMS boxes handle the
  six primary flight and navigation displays: two sets are located in
  front of both the captain and copilot so that they can move from one
  seat to the other, and two central sets of engine parameters are
  shared by the pilots. The primary flight instruments indicate pitch
  and roll attitude, direction, air speed, rate of climb, altitude, etc.
  The AIMS also includes the central maintenance function, which
  receives reports from the 777's other computers and then gathers the
  data into a central maintenance report for the mechanic. Its
  monitoring system gathers data on how other functions are doing, and
  can determine, for example, that an engine is degrading, before it
  actually fails. Other AIMS functions include a data-conversion
  gateway, flight data acquisition, data loading, an Ada conversion
  gateway, and thrust management.
Honeywell's massive effort on the 777
  involved over 550 software developers. The company built the AIMS
  computer as a custom platform based on the AMD 29050 processor. It was
  unique among aviation systems for integrating the other computers'
  functions; in other systems, each function resides in a different box
  [the central maintenance had its own box with its own input/output
  (I/O), its own central processing unit (CPU), etc.]. AIMS combines all
  these functions and shares the CPU and I/O among them: it uses the
  same signals for flight management and for displays, so that the data
  comes in only once instead of twice; one input circuit provides data
  to all of the functions; each of the functions gets a piece of the
  CPU, as in a mainframe computer, where systems use part of the CPU but
  not all of it; and every function is guaranteed its time slot.
  Engineer Jeff Greeson said that "The federated system is obsolete.
  Putting all the functions in one box is a jump ahead in technology
  that we've brought to the industry."
Another innovation is that the
  disk drive can read files formatted for the Microsoft Disk Operating
  System, which provides maintenance with access to the terminal
  communications. The mechanics can transfer files for data loading over
  the airplane bus, because Honeywell built the program to accept new
  data and to change the software. In fact, most of the equipment on the
  airplane has that ability, only a few classic systems do not (such as
  the ground-proximity warning system, which has proven sufficiently
  trustworthy and not in need of change).
Designing a new architecture
  simultaneously with a new language was "quite exciting," Greeson said.
  "The organizational details were difficult to put together." With Ada,
  managers were able to delegate the seven main functions to groups of
  60-100 software engineers. The separate software entities have minimal
  interface with other parts of the software, and not all of the
  software is integrated. By working with loosely coupled pieces, the
  project leaders were able to farm out the functions to other groups.
  The loose integration, however, does not tie the software to the 777
  platform, and will assist in Honeywell's using the code for other
  targets. "We needed the maximum ability to port it to other places,"
  Greeson said.
The data interfaces that do exist between the software
  units are fairly uniform, Greeson said, because Ada helped the
  software engineers to implement certain rules at compilation time.
  "Ada forces you keep it straight there rather than at the lab," he
  said, "where it helped minimize our difficulties in getting it
  integrated and running." Because of the high level of accuracy during
  the compilation, less time was spent on debugging the code. Thus,
  Honeywell's initial study proved correct. "I'm convinced that, because
  of Ada, we had a minimal amount of interface problems, with which we
  would have killed ourselves if we had had C or Pascal," Greeson
  concluded. "It went much smoother than past programs."

This doesn't really answer the question, as this is for one OEM platform (B777) rather than for the industry as a whole.

It's the most informative and detailed source I could find on avionic systems in general. Other sources simply mention that each part has a different isolated chip and is powered by a real-time OS. This article goes way deeper and has some references to the way things were done on older planes as well.

could you try to paraphrase that? or at least highlight the important bits?

@Federico A question asking which programming languages are being used in aviation is useless, because the answer is simple: C, Assembly, ADA, etc. Doesn't teach you much at all knowing what language they use. What matters is the (implementation) details. This article explains why Ada was chosen, for example see: "I'm convinced that, because of Ada, we had a minimal amount of interface problems, with which we would have killed ourselves if we had had C or Pascal.".
It's clearly stated, "interface problems", so Ada was chosen for this reason. That's more informative than just saying "Ada".

what I meant is that if you could try to summarize the article here, and not to have a full copy of it. or at least copy here only the most relevant passages.

@Federico The article isn't very long and has a lot of info, so you can read the whole thing if you're curious about this.


text:
Can a UAV defend itself?
An unmanned aerial vehicle (UAV) was shot down by Turkish aircraft, which made me wonder, whether the Turkish pilots faced any defending moves from the UAV, or once they found it, taking down was a piece of cake?

If drones can attack & defend themselves, one day we'll see drone vs drone dogfights.

Well @kevin, hopefully, they will be more clever than going into a close combat. However, since I am in the Informatics section, I was thinking that leaving the man outside of the aricraft, you could do all sort of brutal maneuvers, without having to worry about the human body taking all the G's. Moreover, it would be very interesting to see what kind of fast and approximate algorithms would run in order to compute the moves in real-time. One femptosecond later and you are hit! On the other hand, taking the human mind out of the play, is sure a minus, since it is likely that never a machine~

@kevin sam vs guided missile/smartbomb?

For those brutal maneuvers you need thrust and wing area. Fitting both in sufficient amount to a drone will cut its loiter time and make it bigger. You will need more of them, and each will be more expensive than a drone designed for observation or strikes. In the end, the drones will continue the cost spiral we have seen for a century in manned fighters. Remember, according to Augustine's Laws, in 2050 the whole US military budget will just buy a single aircraft.

"in 2050 the whole US military budget will just buy a single aircraft." sounds like hot air to me. :)

The existing answers are, of course, correct that most current UAVs do not have much in the way of options for defending themselves against attacks by fighters. This, however, is due primarily to the design missions of most current UAVs, which are reconnaissance and attacking lightly-defended ground targets. Their mission is literally to be an expendable recon or attack aircraft, therefore it doesn't make a lot of sense to invest millions upon millions of dollars in giving them defensive capabilities. Even most normal manned recon aircraft will be short work for a modern jet fighter, with the typical use case being using actual fighters to keep enemies away from them. With a UAV, there's much less incentive to invest heavily in defending it, since they're generally much cheaper aircraft and losing them doesn't mean losing personnel.
Having said all of this, there's nothing inherent about being unmanned that makes an aircraft unable to defend itself. It's just a matter of most current UAVs being designed for use cases where they don't care all that much about defending it, so it's not worth equipping them with lots of excess available lift and thrust, extra sensors, weapons, etc.
There's nothing to say you can't design an unmanned fighter aircraft. For example, if the drone you're trying to shoot down happens to be a QF-16, then you're probably going to have a significantly more challenging mission ahead of you. The QF-16 is exactly what it sounds like - an F-16 that has been modified to act as a UAV. As such, it has the full range of evasive options available to it that you would expect from a normal 4th generation fighter aircraft, since it is one.
The U.S. armed forces use the 'Q' designator to mean that an aircraft is a UAV. This is a 'vehicle type' designator in the 1962 United States Tri-Service aircraft designation system. It is added to the F-16 designation for the unmanned F-16s, but it's also used for all military UAVs, such as the MQ-1 Predator, the MQ-9 Reaper, or the RQ-4 Global Hawk.
QF-16 flying without a pilot:

The QF-16 is exactly what it sounds like - an F-16 that has been modified to act as a UAV.  <- I can't really get this part, but for sure, that's what arised in my mind after leaving home, what if we had a UAV jet fighter? Thanks!

@reirab a QF16 won't try and shoot back.  It's just a fancy target drone.

I got that. I don't see what does Q stand for :/

@gsamaras the Q just means it's a drone conversion.  The US started making QF16's last year because we were running out of F4 airframes to make QF4s for live fire training exercises.

Yes @DanNeely, but I just thought that Q would stand for something reasonable here.

@gsamaras Oh, sorry, the 'Q' designator just means it's unmanned (presumably they just picked a letter because 'U' was already in use for 'utility'.) I'll edit to clarify.

@gsamaras: http://www.wikihow.com/Understand-US-Military-Aircraft-Designations

@DanNeely Yes, targets shooting back would usually be undesirable behavior in aerial combat training (though I suppose it would provide a very big incentive for pilots to take their training seriously.) That's why I said it had the full range of evasive options of a 4th generation fighter, not the full range of combat response options of a 4th generation fighter. :)

Come on - you can't be serious. Who will do the maneuvering? A remote operator? Latency will put him at a distinct disadvantage. Some super-secret artificial intelligence? Sorry, this is still science fiction. I expect we will see this within our lifetime, but currently all drones are more or less defenseless.

@PeterKämpf For clarification, I'm not suggesting that this particular (or any currently existing) drone would have sufficient defensive capabilities to hold its own against an experienced fighter pilot flying a decent jet fighter. My point was just that there's nothing that inherently prevents a UAV from being able to have defensive capabilities; it's more a matter of them not being worth the trade-offs for the use cases of most current UAVs. For whatever it's worth the QF-16s are flown remotely, which does, of course, incur the latency disadvantages you mentioned.

Can a UAV defend itself?
There were attempts to install defense systems on drones but such weapons have not proved effective, at least up to Nov. 2012 (see the article below).
"In 2002 the Air Force fitted some of its early-model Predator drones with short-range Stinger air-to-air missiles. But even with the right weaponry the robots were likely a poor match for enemy fighters, and the Air Force ultimately stripped them of the missiles.
...
The one instance of an air-to-air-armed Predator fighting back against an attacking aircraft resulted in the drone’s destruction.", Source: Predator Drones Once Shot Back at Jets…, authour: DAVID AXE, 11.09.12

If you take a UAV that wasn't designed for air to air combat, and arm it with a missile that wasn't designed for air to air combat, the results are unlikely to be good, that's true.

Short answer: No.
The drone needs sensors, and if the designers were preoccupied with outfitting it for its main mission and did not include anything to detect an incoming threat, a drone is a sitting duck. But even with complete situational awareness it still is close to a sitting duck.
Even with sensors onboard, the options are limited. If you have IR sensors which can spot the cooled IR seeker of an incoming missile, the drone can throw out flares and hope to confuse the missile seeker. However, modern missiles don't go for the hottest target in the sky, but can use the flares to better find their source. This is called "climbing up the stairs" in the vernacular of missile engineers.
The propeller driven drones are a difficult target for IR seekers which are optimized for the hot section of a jet exhaust, so the chances of being hit by an IR guided missile are low. Therefore, it makes sense not to include anti-IR measures - they cost weight and money.
If the threat is another aircraft, radar helps. But putting a radar transmitter on a drone will make it advertise its presence to all suitably tuned receivers. Better to use passive detection, then.
This would be optical or IR sensors which scan the sky and look for rapidly moving patterns. Once found, the drone can fly evasive maneuvers to get away from the threat. But since it is optimized to loiter in a target area, its lift and thrust reserves are most likely much smaller than those of a fighter aircraft. Consequently, maneuvering will only delay the inevitable contact with the enemy. And since the enemy can maneuver faster, he will be first to be able to shoot a missile or a salvo from his gun. Therefore, adding a missile to the drone will also not help. Without onboard radar, this can only be a short-range IR missile, and this needs to be pointed in the right direction before launch.
In the end, the drone will not benefit much from detecting a threat. Yes, it can transmit footage of the enemy closing in, which might help in identifying what happened and could be used in a political dispute, but then drones are built for flying where you don't want to send manned aircraft, so the value of that footage is most likely limited.
This is the price you pay for using low-cost drones instead of fully developed combat aircraft. Once the drone is detected, a determined adversary has little problem taking it out.

Any UAV or UCAV that relies on a data link to a remote operator on the ground or in another aircraft is an obvious non-starter in a high intensity conflict situation, because the enemy will jam the data link. The RAF and the French Air Force are persuing these options, but only because they hope to discredit the unmanned combat aircraft as a concept, thereby enabling them to retain manned combat aircraft in perpetuity.

@J.Southworth: Correct, reliance on a datalink is foolish. But the drone can be made autonomous, and once it is designed for anti-aircraft use, it might very well be superior to a manned platform. But the missing option of last-minute intervention is too hard for the politicians to stomach - therefore the continued reliance on a human in the cockpit. That is still the most adequate solution for low-intensity and asymmetric conflicts.

I don't think political opposition is as much of an issue as opposition from within the services that presently operate manned combat aircraft, who will see their social status as exclusive flying clubs somewhat reduced as a result of the adoption of this technology, when that happens, which it will because the potential increase in military capability that can be obtained is enormous.

Is there any literature on spotting the IR seekers of incoming missiles?

@ErinAnne: When I read about it, that stuff was classified. The physics should be straightforward and since then maybe someone has written a public paper, but I don't know about that.

Because it does not need to incorporate the cockpit structure, pilot and canopy, a UCAV can be smaller and have superior stealth characteristics compared with a manned fighter, other things being equal. In practice this would probably be more significant in air to air combat than superior high g manoeuvering capability, given the ranges at which air to air combat typically takes place, ie 10 to 40 km.
The same rules apply to manned and unmanned aircraft as far as the physics involved in high g manoeuvering are concerned, which is to say that such capability can only be obtained at the expense of range and payload so that in the end a trade-off has to be made. That said, the ability of a UCAV to turn at 18 g could potentially decide the outcome of a dogfight with a manned fighter.

While air to air combat in recent conflicts has taken place at ranges of 10-40km it should be pointed out that a small stealthy UCAV flying at low level would not be detectable by a manned fighter's radar at these distances, and it is not certain that existing visual spectrum, IR or radar based air to air missile guidance would be effective under these conditions either.

A UAV or UCAV can potentially defend itself from attack by other aircraft using ECM, evasive manouvering, the use of weaponry, or a combination of these depending on what is most effective and appropriate in a specific tactical situation.
The development of low cost, solid state, light weight infrared sensors is an important recent advance, which makes it possible to equip an autonomous UCAV with the means to detect threats from missiles and manned aircraft, and react by taking evasive action or, potentially, by counter-attacking the threat.

Millimetric Wave Radar and laser scanning are two more sensor technologies that have obvious applications in UCAVs. If it's possible to make an 81mm mortar bomb that can seek tank targets using MWR (the Royal Ordnance Merlin), it's obviously possible to install a similar MWR system in a small UCAV.


text:
Is there any regulation on how often a private aircraft should be washed?
Is there any regulation on how often a private aircraft, such as a Cessna, should be washed?

I can't imagine why there would be. If there is any, it may be a local airport regulation related to run-off. There is no FAA regulations on washing GA aircraft.

Possible duplicate of Is there any FAA Regulation on the washing of private aircraft such as Cessna Skyhawk or Citation?

This is definitely NOT a duplicate of that question. That question deals with who may or shall wash the plane (and doesn't offer very good answers). This question deals with when it shall be washed.

Yes
...sort of.
There is a general requirement to clean privately operated aircraft as part of the annual inspection. There is no requirement to actually accomplish an annual inspection for a non-flying aircraft stored in a hanger. However, for an aircraft to be airworthy, the mechanic performing the annual inspection shall accomplish the required cleaning as part of the inspection. See 14 CFR 43, Appendix D:

(a) Each person performing an annual or 100-hour inspection shall, before that inspection, remove or open all necessary inspection plates, access doors, fairing, and cowling. He shall thoroughly clean the aircraft and aircraft engine.

Note that the annual inspection is generally the only inspection required of many privately operated aircraft. For aircraft operated commercially for flight instruction or carrying passengers for hire, the 100 hr inspection is also required, which might increase the required cleaning interval.
This regulation applies to all aircraft subject to FAA regulation. There are otherwise no special FAA regulations regarding aircraft washing or cleaning that are applicable to the Great Lakes area.

There may also be manufacturer recommended wash intervals. However, these are not regulations.
Operating certain types of aircraft under an approved maintenance program, the manufacturer could conceivably mandate a wash schedule which would then become mandatory in order to maintain continued airworthiness under that approved maintenance program. Such a mandate could conceivably apply to a specific geographic area based on climate issues. However, in the context of private aircraft, the requirement for such maintenance programs would only apply to certain large, turbojet, or multi-engine turbine aircraft. For this reason, neither of the types originally tagged in your question (Cessna 172 and 208) would be subject to a requirement for an approved maintenance program, though the Citation series originally mentioned in your title would be.
Moreover, if a manufacturer instituted such a change to their maintenance programs, such a change would generally only affect newly approved maintenance programs. Existing maintenance program approvals are "frozen" meaning new manufacturer maintenance program recommendations are recommendations only.

There may be local ordinances that limit water usage or otherwise regulate activities involved in aircraft washing.
Questions regarding EPA regulation are beyond the scope of this site.

I overheard a discussion about a recent "mandate" from Cessna recommending 15/30/45 day washes depending on location. Specifically interested in the great lakes area.

@Ross See my edits my answer. Manufacturer maintenance recommendations are not regulatory and are not generally applicable to private aircraft.

"Yes, there is a general requirement to clean privately operated aircraft at an interval annually."
Got a link for that? Maybe a reg?

@egid There you go, see edits

and of course there's a requirement to have good vision through the windows, which translates into washing them when vision is degraded...

@jwenting Well, to my knowledge there is no explicit regulatory requirement regarding good vision through windows or windscreens outside of current certification standards. Such a requirement might be found in manufacturer recommended maintenance practice, or might be inferred from the requirements of §91.13. However, in the context of this question, "Is there any regulation...", no, there is no such regulation.


text:
Is the F-16 cockpit pressurized?
Is the F-16's cockpit pressurized? If so, why does the pilot receive oxygen from the mask (please state the correct terminology for this)? Is it due to redundancy in case cabin pressurization fails?

Related. Maybe a dupe, despite not being specific to the F-16?

Yes, the cockpit of the F-16 is pressurized. However, there are two types of cabin pressurization:

Isobaric Pressurisation: The system maintains a constant cabin pressure (usually between 2000 and 8000 ft) as the atmospheric pressure decreases. This is used in commercial aircraft.
Hypobaric Pressurisation: In such a system the pressurisation commences at a
given altitude and cabin altitude is maintained at this value until a preset pressure differential is reached. With continued ascent the pressure
differential is maintained. This is used in military aircraft including the F-16, as the weight penalty of the Isobaric system would seriously affect the range.

Provision of supplemental oxygen in the aircraft ensures that the occupant receives increasing quantity of oxygen in the inspired air. The aircraft oxygen system (regulator assembly) ensures that the correct percentage of oxygen is added from the on-board Oxygen reserve to the inspired air in order to maintain lung pO₂ at 103 mm Hg.

Do you have any technical documentation for further reading?

The ATPL Oxford manual of human performance and  limitations should have a chapter on hypoxia or pressurisation

I'm looking for technical documentation that explains why those values, how is oxygen added on this particular aircraft's air system, and many other items related to the F16's pressurisation system. Side effect: most of it would also support your answer.

Might be worth qualifying it as partial pressurization (which is what "hypobaric pressurisation" means).

Does "lung pO₂ at 103 mm Hg" correlate with fingertip spO₂ monitors?

Yes. Like most fighters the cockpit of an F-16 is pressurized, primarily for pilot comfort. Use of an oxygen mask is required equipment for high altitude operations and for emergency situations.  The pilot has a pressure breathing on demand oxygen mask, which is required equipment for high-altitude operations.

I believe that the F-16 can operate in excess of FL500, and generally, a pressure suit is required for operations above FL500, in the event of cabin depressurization.  Use of a mask assures high levels of O2, without causing the risks which a pure O2 environment in the cabin would have (fire primarily).  Survival above a pressure altitude of 50,000 feet is possible, with 100% O2 breathing, but only for a short time. So the pressurized cabin permits F-16 pilots from needing bulky and vision restricting pressure suits in normal operations.

But typically an F-16 operates between 15,000 and FL350 and there may be ops policies restricting altitudes without proper life support gear.

The F-16D flight manual shows a service ceiling of 18km, which is approximately 60,000 feet. Routine maneuvers may exceed the service ceiling.  The cabin pressurization is indicated for flight safety.  If not operational, there are altitude restrictions, as it is required equipment.

No, routine maneuvers in a Viper do NOT exceed that ceiling.  While one might be able to get a Viper up that high on a good day with no ordnance and minimal fuel, that does not represent typical day to day operations which are usually in the teens to mid flight levels (you need thicker air for good engine power and to maneuver).  As for pressurization for flight safety, keep in mind that one minute in an F-16 is the difference between brakes off and rolling and leveling off at FL250 at 400 KIAS.   That’s a hell of an inner ear workout if you don’t have pressurization systems to compensate.